If Amazon is the 800 lb. gorilla in the room, Google is no chimp. A huge company with a lot of clout, ambition, money and a seeming love for words has just entered the eBook market. Google just opened its own ebookstore at http://books.google.com/ebooks and it's chock full of 3 million books (we assume 1 million of that count are their already available public domain works).
Even better, Google has apps ready for Android and the iPhone, iPad and iPod Touch. You can read on your computer and you can read on the Nook and Sony Readers as well as other ebook readers that support Adobe Adept DRM (the most common form of ePUB DRM). That means the Kobo Reader and Alex Reader work too, among others. Nifty.
What's not compatible? Amazon Kindle ereaders are not since they're pretty much the only major ereader line on the planet that eschews ePub and sticks with Amazon's exclusive format.
Prices look similar to B&N, Borders and Amazon in the few minutes I've spent perusing the store's list of best sellers and popular titles (bestseller ebook prices are now often uniform thanks to the switch to agency model ebook pricing).